The third generation partnership project (3GPP) is currently finalizing the standardization the first release of the Long Term Evolution (LTE) concept. In many radio communication networks, as well as in an LTE network, amplify-and-forward repeaters are used to amplify signals in the network. In this manner, for example, the cell size of a radio base station in the network may be enlarged.
Amplify-and-forward repeaters, sometimes also denoted L1 repeaters, are available as on-frequency repeaters (OFR) and frequency translating repeaters (FTR). An OFR receives and transmits on the same frequency, while an FTR receives on a first frequency and transmits on a second frequency, different from the first frequency. An advantage with on-frequency repeaters versus frequency translating repeaters is that the OFR has no frequency domain duplex loss. Since the OFR do not use one frequency for receiving and another frequency for transmitting, no additional frequency resources are consumed. Furthermore, OFR are of particular interest together with OFDM modulation that is used in e.g. LTE. If the delay of an OFR is less than the cyclic prefix then the direct path and the repeated path add in the same way as normal multipath.
A problem when introducing a large number of on-frequency repeaters in a cellular network is that they may interact with each other in an undesired way. In more detail, when a mobile station communicates with a base station via two repeaters, as shown in FIG. 1. The signal that is transmitted from the first repeater, to be received by the base station, is also received by the second repeater. The second repeater amplifies the signal and transmits the signal, which then may be received by the first repeater, which amplifies the signal and sends the signal anew. If the cross coupling between the repeaters is too strong, this will cause a positive feedback interference that destroys the communication between the mobile station and the base station. This feedback problem was initiated by the transmission from the mobile station to the radio base station. Using prior art techniques, this interaction could in many cases not have been detected in advanced.
This type of positive feedback instability problem does not occur in case a mobile station communicates with a base station via two decode-and-forward relays, as shown in FIG. 2. However, also in this situation some knowledge of cross coupling on the radio link between the two relays is typically needed. For example, it may be considered to separate, in the time and/or in the frequency domain, the transmission from the first relay with the reception of the second relay, should the cross coupling be too strong. Also, knowing the inter-relay cross coupling enables more advanced interference coordination schemes, such as joint scheduling of the relay transmissions and receptions. Prior art techniques do not provide means for determining such inter relay couplings.